System and method for maintaining vacuum in superconducting magnet system in event of loss of cooling

ABSTRACT

An apparatus includes: a getter material ( 310 ) disposed within a vacuum chamber ( 210 ) to absorb stray molecules within the vacuum chamber; a thermal mass ( 340 ) disposed adjacent the getter material and in thermal communication with the getter material; a cold station ( 312 ) disposed within the vacuum chamber above the thermal mass; and a convective cooling loop ( 310 ) connected between the thermal mass and the cold station and configured to convectively cool the thermal mass when the cold station is at a lower temperature than the thermal mass, and to thermally isolate the thermal mass from the cold station when the cold station is at a higher temperature than the thermal mass. The thermal mass may be water ice and may be thermally isolated from the walls of vacuum chamber by low loss support links ( 360, 362, 364 ) and/or thermal reflective shielding.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application of InternationalApplication No. PCT/IB2015/059233, filed on Dec. 1, 2015, which claimsthe benefit of U.S. provisional Application Ser. No. 62/091,175 filed onDec. 12, 2014 and is incorporated herein by reference.

TECHNICAL FIELD

The present invention generally pertains to a system and method formaintaining a vacuum in a superconducting magnet system in the event ofa loss of cooling of a cryogenic environment in which thesuperconducting magnet system is deployed.

BACKGROUND AND SUMMARY

Superconducting magnets are used in a variety of contexts, includingnuclear magnetic resonance (NMR) analysis, and magnetic resonanceimaging (MM). To realize superconductivity, a magnet is maintained in acryogenic environment at a temperature near absolute zero. Typically,the magnet includes one or more electrically conductive coils which aredisposed in a cryostat and through which an electrical currentcirculates to create the magnetic field.

There are many ways to maintain the electrically conductive coil(s) ofthe superconducting magnet at cryogenic temperatures so that they remainsuperconducting during normal operation.

In some superconducting magnet systems (for example, so-called “cryofreesystems”) the magnet is maintained in a vacuum space and is cooled by asealed system (e.g., a cold station or cold plate) which is filled witha relatively small amount of cryogenic fluid, for example one or twoliters of liquid helium, so as to transfer heat from the electricallyconductive coil(s) to a cold head which is in turn cooled via acompressor.

In such systems, it is beneficial to provide within the vacuum space agetter which is maintained at a very low temperature (e.g., below 20°K.) so as to absorb stray molecules that may be released into the vacuumspace, as such stray molecules can become a mechanism for heat transfer.In particular, over time the getter material accumulates gas moleculesthat may enter into the vacuum space from very small leaks.

However, it is possible that the cold head may become non-operational,for example due to a malfunction of the compressor, or due to a loss ofAC Mains power for operating the compressor, thereby shutting downrefrigeration of the superconducting magnet system. Such refrigerationshut down may occur during transportation, electrical outages, orequipment failure. In these cases, superconducting magnet systems with asmall thermal heat capacity at low temperatures (e.g., cryofree systemshaving only a small amount of liquid helium inside a sealed system) maywarm up rapidly above 20° K.

Meanwhile, if the getter is allowed to heat up, then the stray moleculeswhich have been captured by the getter may be released into the vacuumchamber or cryostat which holds the superconducting magnet. If thatoccurs, an expensive and time-consuming vacuum pump down of the cryostatmay be required to remove the released molecules.

Accordingly, it would be desired to provide a system and method formaintaining a vacuum in a superconducting magnet system in the event ofa loss of cooling of a cryogenic environment in which thesuperconducting magnet system is deployed.

One aspect of the present invention can provide an apparatus including:a first getter material disposed within a vacuum chamber and which isconfigured to absorb stray molecules within the vacuum chamber; athermal mass disposed adjacent the first getter material and in thermalcommunication with the first getter material; a cold station disposedwithin the vacuum chamber at a height greater than a height at which thethermal mass is disposed; and a convective cooling loop connectedbetween the thermal mass and the cold station and configured toconvectively cool the thermal mass when the cold station is at a lowertemperature than the thermal mass, and to substantially thermallyisolate the thermal mass from the cold station when the cold station isat a higher temperature than the thermal mass.

In some embodiments, the thermal mass can comprise a thermal mass ofwater ice.

In some embodiments, the cold station can be a 4° K. cold station.

In some embodiments, the apparatus can further include: a thermal shielddisposed within the vacuum chamber dividing the vacuum chamber into aninner region and an outer region; and a plurality of first low thermalconductivity support elements which connect the thermal shield to one ormore outer walls of the vacuum chamber, wherein the thermal shield isisolated from the outer walls of the vacuum chamber except for the firstlow thermal conductivity support elements.

In some embodiments, the apparatus can further include: an independentstructure disposed within an inner region of the vacuum chamber; and aplurality of second low thermal conductivity support elements whichconnect the independent structure to the thermal shield, wherein theindependent structure is isolated from the thermal shield except for thesecond low thermal conductivity support elements.

In some embodiments, the apparatus can further include a plurality ofthird low thermal conductivity support elements which connect thethermal mass to the independent structure, wherein the thermal mass isisolated from the independent structure except for the third low thermalconductivity support elements.

In some embodiments, the apparatus can further include a thermallyreflective structure disposed within the first region between thethermal mass and the thermal shield.

In some embodiments, the first getter material can comprise an activatedcharcoal material.

In some embodiments, the apparatus can further include a second gettermaterial separated and apart from the first getter material and disposedadjacent to, and in thermal communication with, the cold station.

In some embodiments, the apparatus can further include a compressordisposed outside the vacuum chamber and connected to the cold station,and configured to conduct heat from the cold station to an exterior ofthe vacuum chamber.

Another aspect of the present invention can provide an apparatus,including: a vacuum chamber having one or more walls enclosing aninterior space of the vacuum chamber; a heat shield disposed within thevacuum chamber, the heat shield defining an inner region of the vacuumchamber within the heat shield and an outer region of the vacuum chamberdisposed between the heat shield and the one or more walls of the vacuumchamber; a superconducting magnet disposed within the inner region ofthe vacuum chamber; a cryocooler configured to cool the superconductingmagnet, the cryocooler providing at least one cold station within theinner region of the vacuum chamber; a getter material disposed withinthe inner region of the vacuum chamber and which is configured to absorbstray molecules within the vacuum chamber; a thermal mass disposedadjacent the getter material and in thermal communication with thegetter material, wherein the thermal mass is disposed at a lower greaterthan a height at which at least one cold station is disposed; and aconvective cooling loop connected between the thermal mass and the coldstation and configured to convectively cool the thermal mass when thecold station is at a lower temperature than the thermal mass, and tosubstantially thermally isolate the thermal mass from the cold stationis at a higher temperature than the thermal mass.

In some embodiments, the apparatus can be a magnetic resonance imaging(MM) apparatus further comprising: a patient table configured to hold apatient; gradient coils configured to at least partially surround aportion of a patient for which the MRI apparatus generates an image; aradio frequency coil configured to apply a radio frequency signal to theportion of a patient and to alter the alignment of this magnetic field;and a scanner configured to detect changes in the magnetic field causedby the radio frequency signal.

In some embodiments, the apparatus can further include: a compressorconnected to remove heat from the cryocooler; and a magnet controllerconfigured to control energization operations for the superconductingmagnet.

In some embodiments, the thermal mass can comprise a thermal mass ofwater ice.

In some embodiments, the apparatus can further include a plurality offirst low thermal conductivity support elements which connect thethermal shield to one or more outer walls of the vacuum chamber, whereinthe thermal shield is isolated from the outer walls of the vacuumchamber except for the first low thermal conductivity support elements.

In some embodiments, the apparatus can further include: an independentstructure disposed within an inner region of the vacuum chamber; and aplurality of second low thermal conductivity support elements whichconnect the independent structure to the thermal shield, wherein theindependent structure is isolated from the thermal shield except for thesecond low thermal conductivity support elements.

In some embodiments, the apparatus can further include a plurality ofthird low thermal conductivity support elements which connect thethermal mass to the independent structure, wherein the thermal mass isisolated from the independent structure except for the third low thermalconductivity support elements.

Yet another aspect of the present invention can provide a methodincluding: providing within a vacuum chamber a thermal mass adjacent toa getter material and in thermal communication with the getter materialto absorb heat from the getter material; cooling the thermal mass with acold station disposed within the vacuum chamber at a height greater thana height at which the thermal mass is disposed, in turn cooling thegetter material, wherein the cooling is performed via a convectivecooling loop connected between the thermal mass and the cold station;and absorbing stray molecules within the vacuum chamber with the cooledgetter material, wherein the convective cooling loop substantiallythermally isolates the thermal mass from the cold station when the coldstation is at a higher temperature than the thermal mass.

In some embodiments, the method can further include cooling the gettermaterial to a temperature below 20° K.

In some embodiments, the method can further include thermally isolatingthe thermal mass from outer walls of the vacuum chamber by a pluralityof low thermal conductivity support elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more readily understood from the detaileddescription of exemplary embodiments presented below considered inconjunction with the accompanying drawings.

FIG. 1 illustrates an exemplary embodiment of a magnetic resonanceimaging (MM) apparatus.

FIG. 2 illustrates an exemplary embodiment of a superconducting magnetsystem which may be included in an MRI apparatus.

FIG. 3 illustrates an exemplary embodiment of a cooling arrangement fora getter which may be employed in superconducting magnet system whichmay be included in an MRI apparatus.

FIG. 4 illustrates some example elements of a method of maintaining avacuum in a superconducting magnet system when cooling is interrupted.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of thepresent invention are shown. The present invention may, however, beembodied in different forms and should not be construed as limited tothe embodiments set forth herein. Rather, these embodiments are providedas teaching examples of the invention. Within the present disclosure andclaims, when something is said to have approximately a certain value,then it means that it is within 10% of that value, and when something issaid to have about a certain value, then it means that it is within 25%of that value.

FIG. 1 illustrates an exemplary embodiment of a magnetic resonanceimaging (MM) apparatus 100. MM apparatus 100 may include a magnet 102; apatient table 104 configured to hold a patient 10; gradient coils 106configured to at least partially surround at least a portion of patient10 for which MRI apparatus 100 generates an image; a radio frequencycoil 108 configured to apply a radio frequency signal to at least theportion of patient 10 which is being imaged, and to alter the alignmentof the magnetic field; and a scanner 110 configured to detect changes inthe magnetic field caused by the radio frequency signal.

The general operation of an MRI apparatus is well known and thereforewill not be repeated here.

FIG. 2 illustrates an exemplary embodiment of a superconducting magnetsystem 200 which may be included in an MRI apparatus. In particular,superconducting magnet system 200 may be one embodiment of magnet 102 inMRI apparatus 100. Furthermore, superconducting magnet system 200 may beone example of a superconducting magnet system for which a system andmethod as described below may be provided for maintaining a vacuum inthe event of a loss of cooling of a cryogenic environment in which thesuperconducting magnet system is deployed. Examples of such a system andmethod will be described in greater detail below.

Superconducting magnet system 200 includes a cryostat 210 and a thermalshield 213 disposed within an enclosed space defined by the outerwall(s) or enclosure of cryostat 210. Thermal shield 213 defines afirst, inner, thermal insulation region 212 a (hereinafter “inner region212 a”) and a second, outer, thermal insulation region 212 b(hereinafter “outer region 212 a”), each of which may be an evacuatedspace where any, gas, liquid, etc. has been removed, comprising a vacuumexcept for the areas occupied by the electrically conductive leads andother components, as described below. Accordingly, cryostat 210 may alsobe considered to be a vacuum chamber.

Superconducting magnet system 200 comprises a superconducting magnetformed as one or more electrically conductive coils 230 and a persistentcurrent switch 240 disposed within inner region 212 a of cryostat 210.Superconducting magnet system 200 also includes: a cryocooler includinga cold head 301, with first and second cooling stages 302 and 303,driven by a compressor 270 to cool cryostat 210; and a magnet controller280 configured to control energization operations for thesuperconducting magnet (i.e., electrically conductive coils 230).

Superconducting magnet system 200 further includes first and secondelectrically conductive leads 201 and 202, third and fourth electricallyconductive leads 203 and 204, fifth and six electrically conductiveleads 205 and 206, and seventh and eighth electrically conductive leads207 and 208.

Superconducting magnet system 200 is a “cryofree” system or sealedsystem wherein cryostat 210 is not provided with means to add cryogenicmaterial (e.g., liquid helium) thereto. Accordingly, superconductingmagnet system 200 includes a cold plate 260 which is connected to, andcooled by, cold head 301. Beneficially, cold plate 260 may be a closedsystem which is filled with and may circulate a cryogenic fluid, forexample liquid helium. In turn, cold plate 360 cools electricallyconductive coil(s) 230 down to a temperature such as about 4° K. whereelectrically conductive coil(s) 230 become superconductive.

Accordingly, in operation, in some example embodiments the temperatureof inner region 212 a may be about 4.2° K. Furthermore, the temperatureof thermal shield 213 may be about 40° K. In that case, if ambient roomtemperature outside cryostat 210 is 300° K., for example, then thetemperature drop across the space defined by inner region 212 a may bein a range from about 4.2° K. to about 40° K., and the temperature dropacross the space defined by outer region 212 b may be in a range fromabout 40° K. to about 300° K.

Persistent current switch 240 may comprise a piece of superconductorwire connected across opposite ends of electrically conductive coil(s)230 via seventh and eighth electrically conductive leads 207 and 208,attached to a small heater.

Magnet controller 280 may comprise a processor and memory, includingnonvolatile memory and volatile memory. The nonvolatile memory may storeprogramming code or instructions (software) for causing the processor toexecute one or more algorithms for controlling operations ofsuperconducting magnet system 280, for example a process of energizingelectrically conductive coil 230. Magnet controller 280 may also beconnected to, and control operations of, switches 215 and 225 and firstand second electrically conductive leads 202 and 204.

In some embodiments, fifth and sixth electrically conductive leads 205and 206 each may be low-loss leads which conduct a minimal amount ofexternal heat to inner region 212 a of cryostat 210. Beneficially fifthand sixth electrically conductive leads 205 and 206 each may be made ofa material which experiences superconductivity at a relatively hightemperature, for example a temperature greater than 50° K., and inparticular at or around 77° K. Beneficially, third and fourthelectrically conductive leads 203 and 204, and fifth and sixthelectrically conductive leads 205 and 206, may be anchored thermally tothermal shield 213. In some embodiments, third and fourth electricallyconductive leads 203 and 204 may be made of copper or brass.

In one variation, superconducting magnet system 200 includes first andsecond switches 215 and 225 disposed in cryostat 210 and configured tobe controlled by magnet controller 280 to selectively connect first andsecond electrically conductive leads 201 and 202 to third and fourthelectrically conductive leads 203 and 204, respectively, duringenergization of electrically conductive (superconductive) coil(s) 230.

In another variation of superconducting magnet system 200, first andsecond electrically conductive leads 201 and 202 may each be retractableleads which are retractable and extendable under control of magnetcontroller 280. Each of first and second electrically conductive leads201 and 202 may be configured in a retracted position to be disposedentirely outside, or substantially entirely outside, of cryostat 210,and in an extended position to extend into cryostat 210 and be engagedwith, and be electrically connected to third and fourth electricallyconductive leads 203 and 204 (for example via contact or transferswitches 215 and 225).

During a startup operation of superconducting magnet system 200, thewire in persistent current switch 240 is heated above its transitiontemperature, so that it becomes resistive. Magnet controller 280 closesswitch 235 and connects an external supply to first and secondelectrically conductive leads 201 and 202 and thereby to electricallyconductive coil(s) 230 (e.g., via fifth, sixth, seventh and eighthelectrically conductive leads 205, 206, 207 and 208). In someembodiments, this may mean that magnet controller 280 extends first andsecond electrically conductive leads 201 and 202 into cryostat 210 to beengaged with, and electrically connected to, third and fourthelectrically conductive leads 203 and 204 (for example via contact ortransfer switches 215 and 225). In other embodiments, this may mean thatmagnet controller 280 closes first and second switches 215 and 225 toconnect first and second electrically conductive leads 201 and 202 tothird and fourth electrically conductive leads 203 and 204.

Electrically conductive coil(s) 230 is initially energized by theexternal power supply passing a current through electrically conductivecoil(s) 230. Since the wire in persistent current switch 240 is beingheated during the startup operation, its resistance is substantiallygreater than that of electrically conductive coil(s) 230, so the currentfrom the external power supply passes through electrically conductivecoil(s) 230. As electrically conductive coil(s) 230 is cooled by coldplate 260, electrically conductive coil(s) 230 becomes superconductingand thus functions as a superconducting magnet.

To go to persistent mode, the current through electrically conductivecoil(s) 230 is adjusted until the desired magnetic field is obtained,then the heater in persistent current switch 240 is turned off After theheater is turned off, the superconductor wire in persistent currentswitch 240 cools to its superconducting temperature, short-circuitingelectrically conductive coil(s) 230, which as mentioned above is alsosuperconducting. Magnet controller 280 then disconnects first and secondelectrically conductive leads 201 and 202 from third and fourthelectrically conductive leads 203 and 204, respectively, and therebydisconnects the external power supply from electrically conductivecoil(s) 230. In some embodiments, this may mean that magnet controller280 retracts first and second electrically conductive leads 201 and 202from cryostat 210 to be disengaged from, and electrically disconnectedfrom third and fourth electrically conductive leads 203 and 204. Inother embodiments, this may mean that magnet controller 280 opens firstand second switches 215 and 225 to disconnect first and secondelectrically conductive leads 201 and 202 from third and fourthelectrically conductive leads 203 and 204. Thenceforth, electricallyconductive (superconducting) coil(s) 230 continue to be cooled by coldhead 301 via the cryogenic fluid (e.g., liquid helium) circulating incold plate 260.

As noted above, stray molecules that may be released into the vacuumspace of cryostat 210 from very small leaks. Such stray molecules canbecome a mechanism for heat transfer. Accordingly, a getter (not shownin FIG. 2) may be provided within cryostat 210, and in particular withininner region 212 a of cryostat which is maintained at a very lowtemperature (e.g., below 20° K.) so as to absorb those stray molecules.However, if this getter is allowed to heat up, for example due to amalfunction of compressor 270, or due to a loss of AC Mains power, orfor other reasons, then the stray molecules which have been captured bythe getter may be released into cryostat 210, which may heat thesuperconducting coil(s) 230 and lead to a quench of the magnetic field.If that occurs, a startup process may have to be repeated. Additionally,an expensive and time-consuming vacuum pump down of the cryostat may berequired to remove the released molecules from cryostat 210.

Accordingly, a system and method have been developed for maintaining thevacuum in a cryostat of a superconducting magnet system, such assuperconducting magnet system 200, in the event of a loss of cooling.Examples of such a system and method will now be described with respectto FIGS. 3 and 4.

FIG. 3 illustrates an exemplary embodiment of a cooling arrangement 300for a getter which may be employed in superconducting magnet systemwhich may be included in an MRI apparatus. In particular, arrangement300 may be included in superconducting magnet system 200, and may beincluded in MRI apparatus 100.

FIG. 3 shows vacuum chamber/cryostat 210, thermal shield 213, innerregion 212 a, outer region 212 b, and cold head 301, including a firststage 302 and a second stage 303, as also shown in FIG. 2. FIG. 3 alsoshows: a first cold station 311 and a second cold station 312 providedby cold head 301; a first getter material 310 and a second gettermaterial 320; a convective cooling loop 330; a thermal mass 340; athermally reflective structure 350; a plurality of first low thermalconductivity support elements 360; a plurality of second low thermalconductivity support elements 362; a plurality of third low thermalconductivity support elements 364; and a low temperature chamber 370.

First cold station 311, first and second getter materials 310 and 320,convective cooling loop 330, thermal mass 340, thermally reflectivestructure 350, second low thermal conductivity support elements 362,third low thermal conductivity support elements 364, and low temperaturechamber 370 are all disposed within inner region 212 a. In operation,inner region 212 a may be maintained at a temperature below 20° K.; insome embodiments at a temperature of about 4.2° K. In that case, firstcold station 311 may be referred to as a 4° K. cold station. Second coldstation 312 and first low thermal conductivity support elements 360 aredisposed within outer region 212 b. In operation, inner region 212 b maybe maintained at a temperature below 70° K.; in some embodiments at atemperature of about 40° K. In that case, second cold station 312 may bereferred to as a 40° K. cold station. Furthermore, low temperaturechamber 370 may be referred to as a 4° K chamber.

In some embodiments, convective cooling loop 330 comprises one or moremetallic (e.g., stainless steel) tubes which are filled with apressurized cryogenic gas, for example pressurized helium gas.Convective cooling loop 330 is connected between thermal mass 340 andfirst cold station 311.

Thermal mass 340 is adjacent to first getter material 310 and in thermalcommunication with the first getter material 310 so as to absorb heatfrom first getter material 310. Here, “adjacent to” is understood tomean in very close proximity to, but not necessarily requiring directcontact with. However, in some embodiments, thermal mass 340 may be indirect contact with first getter material 310. In some embodiments,thermal mass 340 may be thermally coupled to first getter material 310via some intervening coupling element having a very low thermalimpedance, such as a thermally conductive washer, thermal grease, etc.which can effectively thermally couple all or almost all thermal energyfrom first getter material 310 to first cold station 311. Beneficially,thermal mass 340 is capable of absorbing substantial heat energy fromfirst getter material 310 at very low temperatures, for exampletemperatures below about 20° K. Beneficially, thermal mass 340 has asubstantially greater heat capacity than first cold station 311. In someembodiments, thermal mass 340 is capable of absorbing at least 10kilojoules of thermal energy for a temperature change from 4° K. and 20°K. In some embodiments, thermal mass 340 is capable of absorbing atleast 50 kilojoules of thermal energy for a temperature change from 4°K. and 20° K. Significantly, first cold station 311 is disposed withinvacuum chamber/cryostat 310 at a height which is greater than the heightat which thermal mass 340 is disposed. That is, first cold station 311is disposed within vacuum chamber/cryostat 310 above (but notnecessarily directly above) thermal mass 340 with reference to earth.

Beneficially, in the illustrated embodiment, thermal mass 340 isadditionally thermally isolated to some extent from the superconductingmagnet (electrically conductive coil(s) 230) by thermally reflectivestructure 350 which is disposed in inner region 212 a between thermalmass 340 and thermal shield 213 and which partially surrounds thermalmass 340. In some embodiments, thermally reflective structure 350 may beomitted.

Also, in the illustrated embodiment, thermal mass 340 is connected orattached to, but thermally isolated from, low temperature chamber 370 bythe plurality of third low thermal conductivity support elements 364.Beneficially, except for the third low thermal conductivity supportelements 364, thermal mass 340 is otherwise isolated from lowtemperature chamber 370. Furthermore, low temperature chamber 370 isconnected or attached to, but thermally isolated from, heat shield 213by the plurality of second low thermal conductivity support elements362. Beneficially, except for the second low thermal conductivitysupport elements 362, low temperature chamber 370 is otherwise isolatedfrom heat shield 213. Accordingly, low temperature chamber 370 may be anindependent structure within inner region 212 a which may function as astructural support frame for thermal mass 340. Finally, heat shield 213is connected or attached to, but thermally isolated from, the one ormore walls of vacuum chamber/cryostat 210 by the plurality of first lowthermal conductivity support elements 360. Beneficially, except for thefirst low thermal conductivity support elements 350, heat shield 213 isotherwise isolated from the wall(s) of vacuum chamber/cryostat 210.

In some embodiments, first, second, and third low thermal conductivitysupport elements 360, 362 and 364 may each comprise a fiberglass, PVC orceramic band having a high mechanical strength and a low thermalconductivity.

Here, when it is said that elements 360, 362 and 364 have “low” thermalconductivity, it is understood that this may be defined by the maximumthermal energy transfer that the elements may be able to provide for agiven temperature differential across the elements. In particular, insome embodiments, first low thermal conductivity support elements 360collectively may support a maximum heat transfer of no more than a fewwatts for a temperature differential from 40° K. at thermal shield 213to 300° K. at the wall(s) of cryostat 310. In some embodiments, themaximum heat transfer under these conditions may be one watt or less.Similarly, in some embodiments, second low thermal conductivity supportelements 362 collectively may support a maximum heat transfer of no morethan 100 milliwatts for a temperature differential from 4° K. at lowtemperature chamber 370 to 40° K. at thermal shield 213. In someembodiments, the maximum heat transfer under these conditions may be 50milliwatts or less. Finally, in some embodiments, third low thermalconductivity support elements 364 collectively may support a maximumheat transfer of no more than 10 milliwatts for a temperaturedifferential from 20° K. at thermal mass 340 to 80° K. at lowtemperature chamber 370. In some embodiments, the maximum heat transferunder these conditions may be 5 milliwatts or less.

An explanation will now be provided as to how the system or arrangement300 illustrated in FIG. 3 may maintain the vacuum in cryostat 210 for anextended period of time in the event of a loss of cooling from cold head301.

In normal operation, cold head 301 cools first cold station 311 down toa low temperature, for example of about 4° K. Meanwhile, convectivecooling loop 330 cools thermal mass 340, which in turn cools firstgetter material 310. That is, as thermal mass 340 is located at a lowerheight within cryostat 210 than first cold station 311, heat will risethrough convective cooling loop from thermal mass 340 to first coldstation 311. Thus, first cold station cools first getter material 310via first cooling loop 330. When first getter material 310 reaches asufficiently low temperature (e.g., below 20° K.) then it absorbs straymolecules 5 which may be present in cryostat 210. As long as cold head301 is operational, then it maintains first getter material 310 at orbelow the sufficiently low temperature via the actions of convectivecooling loop 330 and thermal mass 340.

When cooling power is lost, i.e., when cold head 301 is no longeroperating—for example due to a malfunction of compressor 270, a loss ofAC Mains power, during transportation of an MRI apparatus (e.g., MRIapparatus 100) in which cryostat 210 is installed, etc.—then thetemperature within cryostat 210 will begin to rise as thesuperconducting magnet (electrically conductive coil(s) 230) heats up.When this happens, then the temperature of first cold station 311 willalso begin to rise.

In general, thermal mass 340 has a sufficiently large thermal capacityand a much lower thermal conductance than first cold station 311. As aresult, the temperature of first cold station 311 will rise more quicklythan the temperature of thermal mass 340. When this happens, and thetemperature of first cold station 311 rises to a higher temperature thanthe temperature of thermal mass 340, then convective cooling loop 330substantially thermally isolates thermal mass 340 from first coldstation 311. A reason for this is that convective cooling loop 330conveys heat via a convection, and thus in the direction in which theheat rises. However, since first cold station 311 is disposed withinvacuum chamber/cryostat 310 at a height which is greater than the heightat which thermal mass 340 is disposed, then heat from first cold station311 cannot be transferred by convective cooling loop to thermal mass 340in the event that the temperature of first cold station 311 is highertemperature than the temperature of thermal mass 340. It is understoodthat a much smaller amount of heat may be transferred from thermal mass340 to cold station 311 via conduction of the metal tubes or pipes ofconvective cooling loop 330. So here, the term “substantially thermallyisolates” is understood to have a specific meaning, namely thatconvection cooling between the two components is prevented, and anyremaining thermal connection is only via conduction.

Meanwhile thermal mass 340 continues to absorb heat from first gettermaterial 310, thereby extending a period of time where first gettermaterial 310 remains active to capture and retain stray molecules 5which may be present within cryostat 310. Furthermore, heat is notreadily absorbed by thermal mass 340 from its environs as it essentiallythermally floats with respect to the temperatures of heat shield 213 andthe walls of vacuum chamber/cryostat 213 by virtue of first, second, andthird low thermal conductivity support elements 360, 362 and 364.Accordingly, the vacuum within cryostat 310 may be maintained for anextended period of time to ride through a time period when thecryocooler is not operating.

In some embodiments, first and second getter materials 310 and 320 mayeach comprise activated charcoal. In some embodiments, second gettermaterial 320 may be provided to capture stray molecules 5 during a timewhen thermal mass 340 and first getter material 310 are still beingcooled down to operating temperature (e.g., below 20° K). In someembodiments, second getter material 320 may be omitted.

In some embodiments, thermal mass 340 may comprise a thermal mass ofwater ice. Water ice is relatively inexpensive, and furthermore has anenthalpy which is twenty times that of copper up to temperatures of 20°K., making it an advantageous material for maintain a temperature ofgetter 320 below a temperature at which it begins to release capturedmolecules in the event of loss of cooling from cold head 301.

FIG. 4 illustrates some example elements of a method 400 of maintaininga vacuum in a superconducting magnet system when cooling is interrupted.

In an element 410 of the method 400, a thermal mass is thermallyisolated from the outer walls of a vacuum chamber. As explained above,in some embodiments this may be done via a plurality of low thermalconductivity support elements.

In another element 420 of the method 400, the thermal mass is disposedwithin the vacuum chamber adjacent to a getter material and in thermalcommunication with the getter material to absorb heat from the firstgetter material.

In another element 430 of the method 400, a cold station disposed withinthe vacuum chamber at a height greater than a height at which thethermal mass is disposed, cools the thermal mass, in turn cooling thegetter material. Here, the cooling is performed via a convective coolingloop connected between the thermal mass and the cold station.

In another element 440 of the method 400, the cooled getter materialabsorbs stray molecules within the vacuum chamber.

In another element 450 of the method 400, when the cold station is at ahigher temperature than the thermal mass—for example when a cryocoolercooling the cold station is not functioning—then the convective coolingloop substantially thermally isolates the thermal mass from the coldstation. As a result, the getter material remains at a low temperaturenecessary for its proper operation for an extended period of time, evenwhen the cooling is no longer being provided to the vacuumchamber/cryostat.

While preferred embodiments are disclosed herein, many variations arepossible which remain within the concept and scope of the invention.Such variations would become clear to one of ordinary skill in the artafter inspection of the specification, drawings and claims herein. Thepresent invention therefore is not to be restricted except within thescope of the appended claims.

What is claimed is:
 1. An apparatus, comprising: a vacuum chamber; afirst getter material disposed within the vacuum chamber and which isconfigured to absorb stray molecules within the vacuum chamber; athermal mass disposed adjacent the first getter material and in thermalcommunication with the first getter material; a cold station disposedwithin the vacuum chamber at a height greater than a height at which thethermal mass is disposed; and a convective cooling loop connectedbetween the thermal mass and the cold station and configured toconvectively cool the thermal mass when the cold station is at a lowertemperature than the thermal mass, and to substantially thermallyisolate the thermal mass from the cold station when the cold station isat a higher temperature than the thermal mass.
 2. The apparatus of claim1, wherein the thermal mass comprises a thermal mass of water ice. 3.The apparatus of claim 1, wherein the cold station is a 4° K. coldstation.
 4. The apparatus of claim 3, further comprising: a thermalshield disposed within the vacuum chamber dividing the vacuum chamberinto an inner region and an outer region; and a plurality of first lowthermal conductivity support elements which connect the thermal shieldto one or more outer walls of the vacuum chamber, wherein the thermalshield is isolated from the outer walls of the vacuum chamber except forthe first low thermal conductivity support elements.
 5. The apparatus ofclaim 4, further comprising: an independent structure disposed within aninner region of the vacuum chamber; and a plurality of second lowthermal conductivity support elements which connect the independentstructure to the thermal shield, wherein the independent structure isisolated from the thermal shield except for the second low thermalconductivity support elements.
 6. The apparatus of claim 5, furthercomprising a plurality of third low thermal conductivity supportelements which connect the thermal mass to the independent structure,wherein the thermal mass is isolated from the independent structureexcept for the third low thermal conductivity support elements.
 7. Theapparatus of claim 4, further comprising a thermally reflectivestructure disposed within the first region between the thermal mass andthe thermal shield.
 8. The apparatus of claim 1, wherein the firstgetter material comprises an activated charcoal material.
 9. Theapparatus of claim 1, further comprising a second getter materialseparated and apart from the first getter material and disposed adjacentto, and in thermal communication with, the cold station.
 10. Theapparatus of claim 1, further comprising a compressor disposed outsidethe vacuum chamber and connected to the cold station and configured toconduct heat from the cold station to an exterior of the vacuum chamber.11. An apparatus, comprising: a vacuum chamber having one or more wallsenclosing an interior space of the vacuum chamber; a heat shielddisposed within the vacuum chamber, the heat shield defining an innerregion of the vacuum chamber within the heat shield and an outer regionof the vacuum chamber disposed between the heat shield and the one ormore walls of the vacuum chamber; a superconducting magnet disposedwithin the inner region of the vacuum chamber; a cryocooler configuredto cool the superconducting magnet, the cryocooler providing at leastone cold station within the inner region of the vacuum chamber; a gettermaterial disposed within the inner region of the vacuum chamber andwhich is configured to absorb stray molecules within the vacuum chamber;a thermal mass disposed adjacent the getter material and in thermalcommunication with the getter material, wherein the thermal mass isdisposed at a lower greater than a height at which the at least one coldstation is disposed; and a convective cooling loop connected between thethermal mass and the cold station and configured to convectively coolthe thermal mass when the cold station is at a lower temperature thanthe thermal mass, and to substantially thermally isolate the thermalmass from the cold station is at a higher temperature than the thermalmass.
 12. The apparatus of claim 11, wherein the apparatus is a magneticresonance imaging (MRI) apparatus further comprising: a patient tableconfigured to hold a patient; gradient coils configured to at leastpartially surround a portion of a patient for which the MRI apparatusgenerates an image; radio frequency coil configured to apply a radiofrequency signal to the portion of a patient and to alter the alignmentof this magnetic field; a wherein the MRI apparatus is configured todetect changes in the magnetic field caused by the radio frequencysignal.
 13. The apparatus of claim 11, further comprising: a compressorconnected to remove heat from the cryocooler; and a magnet controllerconfigured to control energization operations for the superconductingmagnet.
 14. The apparatus of claim 11, wherein the thermal masscomprises a thermal mass of water ice.
 15. The apparatus of claim 11,further comprising a plurality of first low thermal conductivity supportelements which connect the thermal shield to one or more outer walls ofthe vacuum chamber, wherein the thermal shield is isolated from theouter walls of the vacuum chamber except for the first low thermalconductivity support elements.
 16. The apparatus of claim 15, furthercomprising: an independent structure disposed within an inner region ofthe vacuum chamber; and a plurality of second low thermal conductivitysupport elements which connect the independent structure to the thermalshield, wherein the independent structure is isolated from the thermalshield except for the second low thermal conductivity support elements.17. The apparatus of claim 16, further comprising a plurality of thirdlow thermal conductivity support elements which connect the thermal massto the independent structure, wherein the thermal mass is isolated fromthe independent structure except for the third low thermal conductivitysupport elements.
 18. A method, comprising: providing within a vacuumchamber a thermal mass adjacent to a getter material and in thermalcommunication with the getter material to absorb heat from the gettermaterial; cooling the thermal mass with a cold station disposed withinthe vacuum chamber at a height greater than a height at which thethermal mass is disposed, in turn cooling the getter material, whereinthe cooling is performed via a convective cooling loop connected betweenthe thermal mass and the cold station; and absorbing stray moleculeswithin the vacuum chamber with the cooled getter material, wherein theconvective cooling loop substantially thermally isolates the thermalmass from the cold station when the cold station is at a highertemperature than the thermal mass.
 19. The method of claim 18, furthercomprising cooling the getter material to a temperature below 20° K. 20.The method of claim 18, further comprising thermally isolating thethermal mass from outer walls of the vacuum chamber by a plurality oflow thermal conductivity support elements.